Brain VDR Regulate Glucose Balance

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Our lack of understanding regarding how vitamin D regulates glucose prevents its use as an effective diabetes therapy. We have shown that vitamin D can act in the brain to lower glucose levels and that loss of vitamin D receptors (VDR) within the paraventricular hypothalamus (PVH) of the brain are critical for normal glucose levels in obese, but not lean, animals. However, the neurocircuitry/function of VDRPVH neurons, the role of the PVH VDR responding to dietary vitamin D, and mechanisms underlying effects in obese but not lean states are unknown. This raises basic questions regarding how vitamin D receptors mediate glucose balance. We have generated a genetic mouse model with Cre recombinase expression in VDR positive cells (VDRCre). This provides an excellent model to determine the function, necessity, and downstream neuronal targets of VDRPVH neurons. Additionally, utilizing other genetic tools, we can determine if VDR within the PVH are necessary for changes in blood glucose by dietary vitamin D. Last, we can utilize these tools to determine the mechanisms underlying weight-specific effects of vitamin D in the brain on glucose regulation. The objective of this grant is to determine the mechanisms of vitamin D in the brain on glucose balance. We hypothesize that VDR regulate glucose levels through distinct neuronal circuits and through genomic effects in PVH neurons. The central hypothesis will be tested by three specific aims: 1) identifying neuronal mechanisms for PVH VDR positive neurons; 2) determining if PVH VDR are required or sufficient for dietary-vitamin D changes in glucose homeostasis; and 3) establishing mechanisms for the glucose-protective effect of vitamin D in an obese model. In Aim 1, we will use chemogenetics, single-cell genomics, and immunohistochemistry to determine the function, identity, and circuitry of VDRPVH neurons. In Aim 2, we will use different dietary manipulations of vitamin D to test if PVH VDR are necessary for high-vitamin D induced glucose improvements. Additionally, we will determine if central administration of active vitamin D can overcome deleterious effects of low dietary vitamin D on glucose balance. In Aim 3, we will determine how obesity alters the transcriptomic and neuronal activation response to active vitamin D (1,25D3). Additionally, we will determine if there are differences in VDR expression or VDR+ neuronal number in obese vs. lean states. The research proposed is innovative, because it investigates the function of a novel neuronal population (VDRPVH) on glucose tolerance, using a novel mouse model. The proposed research is significant because it is expected to identify new paradigms to understand vitamin D action, as well as possibly identifying a novel circuit in the PVH with critical glucose-regulating properties. Results from this research may ultimately explain some of the variance in clinical trials utilizing vitamin D as a therapy and provide critical information to advance the use of vitamin D as a therapeutic agent. Altogether, I envision that the completion of this proposal will move this research towards the long-term goal of understanding how to utilize vitamin D as an effective therapy for type 2 diabetes.